riscv: sifive: fix Kconfig errata warning

The SOC_SIFIVE Kconfig entry unconditionally selects ERRATA_SIFIVE.
However, ERRATA_SIFIVE depends on RISCV_ERRATA_ALTERNATIVE, which is
not set, so SOC_SIFIVE should either depend on or select
RISCV_ERRATA_ALTERNATIVE. Use 'select' here to quieten the Kconfig
warning.

WARNING: unmet direct dependencies detected for ERRATA_SIFIVE
  Depends on [n]: RISCV_ERRATA_ALTERNATIVE [=n]
  Selected by [y]:
  - SOC_SIFIVE [=y]

Fixes: 1a0e5dbd ("riscv: sifive: Add SiFive alternative ports")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: linux-riscv@lists.infradead.org
Cc: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>